Device having a switch comprising a chromium layer

ABSTRACT

A device having a switch comprises a chromium layer and an adjacent semiconductor layer. The fraction of voids in the chromium layer is less than 10%, preferably less than 2%. The chromium layer in the device comprises traces of neon with a concentration of less than 0.1 at. %. 
     Chromium layers are deposited on a substrate by means of a sputter deposition process. By using neon as the working gas at pressures of less than 1 Pa, preferably in the range from 0.2 Pa to 0.5 Pa, the sputter-deposited chromium layers are substantially free of internal stress and have a density which is approximately equal to that of bulk chromium.

The invention relates to a device having a switch comprising a chromiumlayer and an adjacent semiconductor layer.

The invention also relates to a method of depositing chromium layers bymeans of sputtering in a working gas atmosphere comprising a group VIIIelement.

Chromium layers are used as a contact layer (at the bottom) of asemiconductor layer, for example an α-SiN_(x) -H layer, in a switch in adevice, for example in a device such as a LCD-device or plasma-device.The switching voltage of the switch in the device is preferably as lowpossible.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the switchingcharacteristics of the switch in the device.

According to the invention, the device having a switch comprising achromium layer and an adjacent semiconductor layer is characterized inthat the fraction of voids in the chromium layer is less than 10%.Preferably, the fraction of voids in the chromium layer is less than 2%.

The interface between the chromium layer and the semiconductor layerfunctions as a Schottky barrier. The inventors have realized that theexistence of a (thin) oxide layer at the interface between the chromiumlayer and the semiconductor layer is deleterious to the operation of theswitch because it comprises a (diffusion) barrier to the injection ofelectrons from the chromium layer into the semiconductor layer.

To establish an optimal contact between the chromium layer and thesemiconductor layer a flat and smooth surface of the chromium layer isrequired. With an increasing fraction of voids in the chromium layer thestructure of the surface of the chromium layer becomes more and morerough. For example, a metal layer which during fabrication develops astrong columnar growth will have a rough surface structure. Such a roughchromium layer with a relatively high specific surface is easieroxidized and the less smooth the surface of the layer is, the thickerthe (effective) thickness of the (native) oxide layer becomes. Theelectrons which have to be injected in the semiconductor layer (via aprocess of tunnelling through a Schottky barrier) are hampered by thisrelatively thick oxide diffusion barrier.

It is noted that in this invention by the wording switch an electronicelement is meant by which a current can be controlled. With the wordingvoid fraction f_(v) it is meant the relative density ρ of the materialwith respect to the bulk density ρ_(bulk) of the material, i.e.,##EQU1##

Conventionally chromium layers as contact layer in switches of a deviceare deposited by means of a sputtering technique. The known sputteringtechniques are, however, less suitable for the present purpose. Theinventors have realized that the problem with oxide (barrier) layers areof a more general type.

A method for depositing chromium layers as described in the openingparagraph is known from Japanese Patent Application 5-65630. In thispatent application an element of group VIII of the periodic table ofelements is used as a working gas in a sputter deposition device. Whilethe pressure of the above-mentioned working gases is controlled in therange from 15 to 25 mTorr (i.e. approximately from 2.0 to 3.3 Pa),chromium layers sputtered from a Cr target are deposited on a glasssubstrate. By confining the pressure of the working gas in the sputterdeposition device to the above mentioned range, the resulting Cr layersare said to be dense and practically free from internal stress.

During sputtering, voids and/or molecules of the sputtering gas tend tobe included in the sputter-deposited layers. This leads to porous filmswith a less than optimal density, i.e. a density which can besubstantially lower than the bulk density. For certain applications theincorporations of gaseous impurities have a detrimental effect on thefunctionality of the deposited layers.

It is an object of the present invention to provide a method ofsputtering Cr layers on a substrate, which layers are substantially freeof internal stress and have a density which is approximately equal tothat of bulk chromium.

According to the invention, the method of depositing chromium layers ischaracterized in that the group VIII element is neon and in that theworking-gas pressure of neon during sputtering is less than 1 Pa.Preferably, the working-gas pressure of neon during sputtering lies inthe range from 0.2 Pa to 0.5 Pa.

Surprisingly, it was found that using Ne in this pressure range, whichis far below the pressure range in the Japanese Patent Applicationmentioned above, and for which pressure range according to the JapanesePatent Application a high internal stress is to be expected, Cr layerswere obtained having a bulk-like density and virtually no stress.

A further advantage is that under these conditions, as compared to theknown conditions, the mean free path in the sputtering gas issubstantially longer, enabling an increase of the distance between thesputtering target and the substrate, which leads to an improved controlof the deposition process and more homogeneous layers.

According to the invention, the device having a switch comprising achromium layer and an adjacent semiconductor layer is characterized inthat the chromium layer comprises traces of neon with a concentration ofless than 0.1 at. %. In general, a concentration of neon of more than0.005 at. % is present in the chromium layer. Concentrations of neonentrapped in a chromium layer can, for example, be measured with a(calibrated) electron microprobe.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 shows an example of a diagrammatic cross-sectional view of aswitch in a device according to the invention;

FIG. 2 shows an example of a diagrammatic cross-sectional view of asputter deposition device;

FIG. 3 shows the internal stress in Cr films sputter deposited in Ar andin Ne as a function of the working-gas pressure;

FIG. 4 shows the real part of the complex dielectric function of Crfilms sputter deposited in Ar and in Ne as a function of the photonenergy.

The drawings are purely diagrammatic and not drawn to scale, withcorresponding parts generally bearing the same reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 an example of a diagrammatic cross-sectional view of a switchin a device according to the invention is shown. On a substrate 21, forexample made of ITO (indium tin oxide) covered glass, a first metalcontact layer 22, for example of chromium, is deposited. Alternativematerials for the first metal contact layer 22 are gold or aluminium. Ontop of the first metal contact layer 22 a semiconductor layer 23 isdeposited. The semiconductor layer 23 comprises for example, amorphoussilicon, poly-crystalline silicon or amorphous siliconnitride, forexample α-SiN_(x) -H. A second metal contact layer 24 is deposited ontop of the semiconductor layer 23. Suitable materials for the metalcontact layer 24 are molybdenum or any other refractory metal.

In general, an interface oxide layer 26 is present between the firstmetal contact layer 22 and the adjacent semiconductor layer 23. Theinterface between the first metal contact layer 22 and the semiconductorlayer 23 as well as the interface between the semiconductor layer 23 andthe adjacent second metal contact layer 22 function as Schottkybarriers. The inventors have realized that the existence of a (thin)oxide layer at the interface between the first metal contact layers 22and the semiconductor layer 23 is deleterious to the operation of theswitch because it comprises a (diffusion) barrier to the injection ofelectrons from the first metal contact layer 22 into the semiconductorlayer 23.

In FIG. 2 an example of a diagrammatic cross-sectional view of asputter-deposition device is shown. The sputtering system, preferably amagnetron sputtering device, comprises a vacuum chamber 1 which can befilled with a working gas via entrance 2, and which is connected to avacuum system via entrance 3. The sputtering system comprises at leasttwo electrodes 5, 7. One of the electrodes is the cold cathode 5, whichis connected to a high-voltage supply via connection 9. The frontsurface of the cathode 5 is covered with the target material 6. Thesubstrate(s) 8 are placed on the anode 7. A glow discharge 10 (plasma)is maintained between the electrodes 5, 7.

In FIG. 3 the internal stress (σ_(int) in GPa) is shown as a function ofthe working-gas pressure (p in Pa) for Cr films deposited in asputter-deposition device using Ar (Cr/Ar) or Ne (Cr/Ne) as the workinggas. In general, the deposited layers are under tension (T) for positivevalues of the internal stress, whereas the deposited layers are undercompression (C) for negative values of the internal stress. At values ofthe internal stress around 0 GPa, the deposited layers are practicallyfree of internal stress. If Ar is used as the working gas in the sputterdeposition device, a transition of the internal stress between tensionand compression is observed at working-gas pressures in the range from 2to 3 Pa (see FIG. 3 Cr/Ar), in accordance with the above mentionedJapanese Patent Application 5-65630. However, within the framework ofthe invention it has been found that Cr layers that are sputterdeposited at Ar working-gas pressures under the known conditions containmany voids (up to a volume fraction of 20 to 30%) leading to a reduceddensity of the Cr layers. This low density is caused by the low thermalvelocity of the Cr atoms close to the substrate. It is the aim of thepresent invention to sputter Cr layers on a substrate which aresubstantially free of internal stress and exhibit an increased density.

Cr films sputter deposited in an Ar atmosphere with a working-gaspressure ranging from approximately 0.1 to 2.0 Pa exhibit only tensilestress (internal stress in the range from +1 to +2 GPa). In addition,the microstructure of Cr films sputtered in Ar has relatively manyvoids.

In order to obtain Cr films with minimal internal stress and withbulk-like properties, a method according to the invention ischaracterized in that Ne is used as the working gas in the sputterdeposition device and the Ne working-gas pressure is less than 1 Pa.

Japanese Patent Application 5-65630 describes in an example the use ofAr as the working gas in a sputter-deposition device at a pressureranging from 15 to 25 mTorr (i.e. approximately from 2.0 to 3.3 Pa).According to this Japanese Patent Application, the same pressureconditions should be used for other gases of group VIII of the periodictable of elements, i.e. operating the sputter-deposition device atworking-gas pressures in the range from 2 to 3 Pa. While Ne is a noblegas element with a smaller atomic radius than Ar, one could select atleast the same or possibly a slightly higher working-gas pressure todeposit the layers, i.e. pressures above at least 2.0 Pa but below 3.3Pa. However, the inventors have realized that this would have anunfavourable effect on the number of voids and/or molecules of thesputtering gas incorporated in the deposited layers, which would lead toa substantially decreased density with respect to the bulk density. If,during depositing of Cr films in a sputter deposition device, the Neworking-gas pressure is selected to be less than about 1 Pa, the densityof the deposited layers is or approaches that of bulk chromium and, inaddition, the deposited Cr layers are practically free from internalstress. It was found that Cr layers with minimal internal stress areobtained at Ne working-gas pressures during sputtering in the range from0.2 Pa to 0.5 Pa (see FIG. 3 Cr/Ne). In this working-gas pressure rangethe internal stress of Cr layers deposited in a sputter-depositiondevice with Ne as the working gas was in the range from -0.5 to +0.5 GPa(see the dotted lines in FIG. 3), which differs substantially from theabove mentioned Japanese Patent Application in which in said pressurerange the stress should exhibit a maximum.

FIG. 4 shows results of measurements obtained by spectroscopicellipsometry of bulk chromium as well as of Cr films deposited underdifferent working-gas conditions. It is well known that measurements ofthe optical constants by means of spectroscopic ellipsometry are verysensitive to the microstructure and the density of the deposited layersas well as to the conditions under which the layers are deposited. InFIG. 4 the real part (ε₁) of the complex dielectric function is plottedas a function of the photon energy (E_(ph) in eV) of Cr filmssputter-deposited in Ar (Cr/Ar) and in Ne (Cr/Ne). The working-gaspressure of Ar in the sputter-deposition device was 1.0 Pa whereas theworking-gas pressure of Ne was 0.5 Pa. The solid line in FIG. 4represents the real part of the complex dielectric function as afunction of the photon energy of bulk Cr (Cr/bulk), as can, for example,be found in E. D. Palik: "Handbook of Optical Constants of Solids"(Academic Press Inc., New York, 1985). A great similarity is observedwhen comparing the results of ellipsometric measurements for bulk Crwith those for Cr films deposited under Ne working-gas conditions, whichshows that the density of the Cr/Ne films is substantially bulk-like.

The results of ellipsometric measurements for Cr films deposited underAr working-gas conditions significantly deviate from the results forbulk Cr, indicating that the density of Cr/Ar layers is substantiallylower than that of bulk Cr. By means of the so-called `Effective MediumTheory` as can be found, for example, in the article by B. T. Sullivanand R. R. Parsons: "A spectro-ellipsometric investigation of the effectof Ar partial pressure on sputtered Palladium films" (J. Vac. Sci.Technol. A5 (1987) 3399-3407) effects of structural variations on theoptical constants of deposited materials can be simulated. Using thistheory to simulate Cr layers containing different vacuum fractions, itwas found that the incorporation of about 30% voids into a deposited Crlayer results in a curve (Cr/voids) for the real part of the complexdielectric function as a function of the photon energy, showing aconsiderable similarity with the curve for Cr films sputter-deposited inan Ar working-gas atmosphere, which indicates that the density of Crlayers sputter-deposited in Ar is far from bulk like. As was mentionedabove, the Ar working-gas pressure used to deposit these Cr layers was1.0 Pa, which is well below the working-gas pressure as preferred in theprior art (i.e. above at least 2.0 Pa), which shows that the density ofthe Cr layers as obtained in the prior art should be substantially lessthan that of bulk Cr.

Neon is the only element in group VIII of the periodic table of theelements that can be selected as a working gas in a sputter-depositiondevice employing a Cr target by which Cr layers can be made that aresubstantially free of internal stress and have a density which isapproximately equal to that of bulk chromium.

For the chromium layers with a void fraction less than 10%, thefollowing relation between the minimum in the real part of the complexdielectric function ε_(min) and the minimum in the real part of thecomplex dielectric function of bulk chromium ε_(min) ^(bulk) holds:

    |ε.sub.min -ε.sub.min.sup.bulk |<3

What is claimed is:
 1. A device having a switch comprising a chromiumlayer and an adjacent semiconductor layer, the chromium layer having afraction of voids of less than 10%.
 2. The device of claim 1, whereinthe fraction of voids in the chromium layer is less than 2%.
 3. Thedevice of claim 2, wherein the chromium layer comprises traces of neonwith a concentration of less than 0.1 at. %.
 4. The device of claim 1,wherein the chromium layer comprises traces of neon with a concentrationof less than 0.1 at. %.
 5. A device as claimed in claim 1, wherein thechromium layer is produced by sputter deposition using neon in aworking-gas atmosphere, the working-gas pressure of neon duringsputtering being less than about 1 Pa.
 6. A device as claimed in claim5, wherein the working-gas pressure of neon during sputtering comprisesa pressure in the range from approximately 0.2 Pa to approximately 0.5Pa.
 7. A device as claimed in claim 5, wherein the working-gas pressureof neon during sputtering comprises a pressure of approximately 0.5 Pa.